The present invention relates generally to the fabrication of printed circuits; more particularly to compositions for microetching copper surfaces in the course of fabricating a printed circuit; and still more particularly to printed circuit fabrication processes employing the copper microetching compositions in admixture with through-hole conditioners.
There are a wide variety of techniques employed for fabricating printed circuits, be they single-sided or double-sided circuits, multilayer circuits, rigid or flexible circuits, or the like. In a great many fabrication techniques, a necessary step in the fabrication sequence involves the controlled microetching of a copper surface, generally for purposes of preparing the copper surface for some subsequent step, such as the provision thereon of a coating layer of metal, organic resist or masking material, or the like.
This surface microetching is in distinct contrast to the process of "copper etching" which is common to all subtractive processes for fabricating printed circuits, i.e., wherein the object is the complete removal (etching away) of selected copper areas from the underlying substrate surface. Instead, the general aim of copper microetching is the controlled removal of only a very small amount of copper from the surface (e.g., removal of only about 0.5 to 3 microns thickness from a copper layer of 30-40 microns thickness), and in a manner which results in a remaining exposed copper surface which is topographically altered (e.g., microroughened) as compared to the original copper surface.
By way of brief example, circuitry innerlayers for use in fabricating multilayer printed circuits are typically prepared from a copper foil-clad insulating substrate material (e.g., epoxy, polyimide) which is then patterned, in the positive of the desired circuitry pattern, with an organic etch-resistant material. Copper areas not covered by the resist are then chemically etched away, and the resist then removed to reveal the selective copper circuitry pattern. In assembling a multilayer printed circuit, one or more such circuitry innerlayers are stacked in alternating array with one or more pre-preg layers comprised of partially cured resinous material, and the composite then subjected to heat and pressure to cure the resin and provide a multilayer circuit structure in which the innerlayer circuitry layers are firmly bonded to resin layers. In this regard, it is commonplace to form a conversion coating of copper oxide on the copper surfaces of the innerlayer circuits, before the laminating process, to promote adhesion between the innerlayer circuits and cured resins (see, e.g., U.S. Pat. Nos. 4,409,037 and 4,844,981), and it generally is necessary to first microetch the copper surfaces before formation thereon of the copper oxide conversion coating.
Another fabrication sequence in which a copper microetch is employed is in the fabrication of double-sided or multilayer printed circuits having metallized (plated) through-holes. In a multilayer printed circuit, for example, the multilayer composite (whose outer layers are copper foil-clad substrate material) is provided with drilled through-holes and is then subjected to steps for providing the through-hole surfaces with a metal coating, such as by electroless plating. In the course of such plating, metal also is deposited over the copper foil surfaces, and it is commonplace in the process to microetch these foil surfaces (as well as any copper innerlayer surfaces or edges exposed at the through-hole) so that they will more adherently receive the subsequent metal coating.
The foregoing are just a few examples of the many sequences in which there is employed a microetch of a copper surface in the course of fabricating a printed circuit. Other examples include the microetching of copper surfaces to improve adhesion thereto of organic plating or etch resists, microetching of copper surfaces to improve adhesion thereto of other metal deposits (e.g., electrolytic copper, tin-lead etch resists, etc.), microetching of copper surfaces to improve adhesion thereto of solder masks, and the like.
The copper microetchants heretofore employed in the art are acidic compositions, and the most widely employed of these are mixtures of hydrogen peroxide with mineral acids, and mixtures of acids with other strong oxidizers such as the persulfates. While these compositions are effective for achieving the desired controlled microetching of copper surfaces, they are not without disadvantages. One problem is that the compositions are highly unstable, and break down continuously even upon sitting idle. Another problem is that these acidic compositions contribute to the so-called "pink ring" phenomenon when used as copper microetchants in sequences for metallizing through-holes in multilayer printed circuits. As earlier noted, copper oxide conversion coatings are commonly employed as adhesion promoters for the innerlayer circuitry-to-resin bonding needed to provide an integrally bonded multilayer circuit. When through-holes are drilled, the edges of innerlayer circuits are there 10 exposed and the processing chemicals for metallizing the through-holes thus have access to the innerlayer edge areas. The copper oxide adhesion promoter is generally soluble in acid, so acidic processing chemicals for through-hole metallization (such as the acidic copper microetchants) promote localized dissolution of the oxide layer about the periphery of the hole. The dissolution evidences itself as a "pink ring" by virtue of the pink color of the underlying copper metal which is exposed as the copper oxide dissolves therefrom. This localized loss of adhesion promoter (which also can occur at non-hole edge areas of the circuit) can in turn lead to localized delamination in the multilayer circuit, an essentially fatal defect.
While an alkaline copper microetchant would be advantageous in this regard, no such suitable compositions have heretofore been available. Alkaline solutions are known and available for completely etching copper from an underlying substrate surface, as earlier described (also sometimes referred to as "final" etchants or "primary" etchants), such as ammoniacal copper solutions. However, these known solutions in particular, as well as most other known final etchants such as acid-based ferric or copper chloride solutions, are highly unsuitable for copper microetching. In particular, these etchants are extremely difficult to control (i.e., to remove only a very small portion of copper as opposed to a complete removal of all copper); more fundamentally, even to the extent a degree of control can be achieved so as to obtain less than complete removal of copper, the copper surface they produce is undesirably smooth and lacks the microroughened topography essential to the subsequent fabrication steps.
Another drawback of the known acid-based copper microetchants is related to a recent trend in the printed circuit fabrication art towards attempts to reduce the number of separate processing steps needed in the fabrication sequence. In particular, sequences for the metallization of through-holes traditionally require a number of separate steps such as (a) contact of the printed circuit board and hole surfaces with an oxidizing agent to remove resin smear from innerlayer circuitry edges exposed at the hole surfaces and/or to etch resin back from innerlayer edges at hole surfaces and/or to topographically alter the resin surfaces of the holes; (b) contact with a neutralizer to neutralize residual oxidizing species from step (a); (c) contact with a so-called through-hole conditioner which promotes adherence to the through-hole surfaces of subsequently-applied catalyst species; (d) copper microetching; (e) contact with species catalytic to subsequent metal depositing; and (f) contact with a suitable metal depositing solution. Efforts to reduce the number of such steps, by combining together in one composition components which will thus effect in one step two or more of the necessary functions, have included combination of neutralizing and microetching steps, and combination of neutralizing, conditioning and microetching steps. See, e.g., U.S. Pat. No. 4,751,106 and 5,104,687, both incorporated herein by reference.
While the goal of these patents is commendable, the efforts to date are plagued with problems. Specifically, in the aforementioned U.S. Pat. No. 4,751,106, the combination of neutralizing and microetching functions necessarily is constrained to the use of an acidic hydrogen peroxide solution, i.e., the conventional acidic copper microetchant as earlier discussed, which also serves as a neutralizer (reducing agent) for permanganate species utilized in the preceding oxidizing step. In the aforementioned U.S. Pat. No. 5,104,687, the combination of neutralizing, conditioning and microetching steps also is strictly constrained to use of acidic hydrogen peroxide (for neutralizing and microetching), and is further constrained to use of a very specific class of throughhole conditioners, i.e., cationic polyelectrolytes. The requirement for acidic hydrogen peroxide in these processes necessarily brings into play the stability problems and pink ring problems earlier discussed. The further requirement for cationic polyelectrolyte, which according to U.S. Pat. No. 5,104,687 is the only through-hole conditioner compatible with its combined neutralizer-microetchant system, essentially prevents the process from being of any commercial interest, because these conditioners have serious drawbacks in metallization processes per se, and particularly as contemplated in the proposed combination of materials and steps.
As such, the continued reliance in the art upon acidic microetchants for copper in printed circuit fabrication techniques not only requires one to deal with the instability and pink ring problems, but also severely limits possibilities for economizing fabrication techniques through process step reduction.
For the reasons given, a substantial need exists for provision of an alkaline composition suitable as a copper microetchant in this environment.